Cats generally try to hide signs of illness so they don't appear weak to potential predators. This means, even if Fluffy plays and eats, she may be ill and trying not to show it. Meanwhile, though lethargy and appetite lack are common symptoms of illness, they don't necessarily accompany all feline diseases. Other symptoms and subtle behavioral changes, besides not eating or playing, indicate Fluffy's health is compromised, too.
Increase in Activity or Appetite
Observe your cat for signs of change in her appetite or activity level. If your normally mellow kitty suddenly begins bouncing off the walls while playing with her toys, or begins voraciously eating more than usual, she may have an illness of some kind. Both diabetes mellitus and hyperthyroidism can increase your feline friend's activity level and her appetite, according to CatChannel.com. With such diseases, you might notice weight loss even while your cat is eating plenty of food. Infestation with intestinal parasites or gastrointestinal issues also can increase your cat's appetite, while possibly not affecting her desire to play.
Symptoms of Illness
A lack of appetite and a disinterest in playing are only two signs of illness in a cat. Many illnesses don't actually result in these symptoms, at least not initially. Other symptoms that indicate your cat is sick include eliminating outside the litter box, poor coat condition, changes in weight, excessive vocalization, changes in sleep patterns or changes in social interactions with you. You might notice Fluffy vomiting frequently or experiencing diarrhea, both of which indicate a variety of medical conditions. One or more of these and other symptoms suggests your playful kitty with a normal appetite is ill.
Not all symptoms of your kitty's illness are obvious. While your sick kitty might eat, you may notice that she eats less than usual, even just slightly less. Your kitty may play at times but seem lethargic at others. Her illness isn't going to completely eliminate her energy or appetite, especially in its initial stages. Just like with people, Fluffy may feel okay for short periods of time but then become weakened by her condition, especially after physical activity. If, overall, your kitty just doesn't seem to be herself, if something seems "off" about her behavior, take her to the vet for a checkup.
Your cat may play or eat when she's sick, and she may not. Any number of medical conditions can lead to a waning appetite and lethargy, including dental issues, kidney disease, cancer or a simple upper respiratory infection. Without proper veterinary treatment, your kitty will continue to avoid food; this alone could lead to further medical problems, including rapid weight loss and a condition known as hepatic lipidosis. Hepatic lipidosis affects a cat's liver and can eventually result in liver damage or even liver failure. To avoid such issues, visit the vet as soon as you notice your kitty isn't eating or participating in any of her normal activities, including play.